isi 2011 variance doubt

[sneha]

I have tried solving dis question by putting values till n=3.I got the mean N/2 but unable to solve it for variance.
Plz Help

Also,if anybody could suggest a better method for solving it

[happy]

ans c

p(x=r) = nCr / 2^n = ncr (0.5)^n = nCr (0.5)^r  (0.5)^n-r

binomial distribution with p = 0.5

[sneha]

Thnx!!